Method of making electric power cable

ABSTRACT

D R A W I N G UNDULATORY DRAIN WIRES ARE EMBEDDED IN A CABLE JACKET BY PAYING THEM INTO AN EXTRUDER DIE DURING THE EXTRUSION OPERATION.

June 1, 19,71 v v.J. PLATE ETAI- 3,582,417: v I A METHOD 0F MAKING ELECTRIC POWER CABLE lOriginal Filed Drec, 27, 1967 I l 'l l2 Pfg/,

BY um@ mm 'um United States Patent O U.S. Cl. 156-51 1 'Claim ABSTRACT OF THE DISCLOSURE Undulatory drain wires are embedded in a cable jacket by paying them into an extruder die during the extrusion operation.

CROSS REFERENCE TO RELATED APPLICATION This application is a division of application Ser. No. 692,893, tiled Dec. 27, 1967, now Pat. 3,474,189.

BACKGROUND OF THE INVENTION In the distribution of electric power it has long been the practice to construct electric cables with a layer of semiconducting bedding tapes over the insulation, surrounded by copper shielding tapes, and protected by an abrasion resistant non-conducting jacket. It has also been known to use buried cables with semiconducting outer abrasive resistant jackets overwound with helical neutral or drain wires. In the manufacture of such cables a separate operation is required to apply the outer wires and the diameter of the cable is increased by their presence, while the drain wires themselves are exposed to corrosion.

We have invented a cable providing substantially all of the protection against abrasion of the known cables but with a reduced overall diameter and lower cost of manufacture. Reduction in the diameter of a cable ismuch to be desired because it permits longer lengths of continuous cable to be wound on a shipping reel, with a resulting reduction in the number of field splices required. Smaller cables also require less space in ducts and conduits with a resulting lowering of installation costs. The reduced cost of our cable stems from the omission of the operation of winding the drain wires. In known types of cable the drain wires had to be helically applied in order to be held onto the cable core and to permit the necessary flexibility that is required of cables that are wound on shipping reels.

Our invention, as will be more fully explained hereinbelow, also affords an increased protection against mechanical damage or penetration of the cable.

Furthermore, our cable affords more intimate contact and lower electrical resistance between the semiconducting cable jacket and the drain wires.

SUMMARY OF THE INVENTION The cable of our invention comprises a conductor, a wall of insulation surrounding the conductor, a thick jacket of semiconducting polymeric material directly surrounding the wall of insulation, and a plurality of undulating drain wires imbedded, for at least a major portion thereof, in the jacket. The drain wires are laid substantially in line with the axis of the cable rather than being wrapped helically around it. In one embodiment the undulations of the drain wires are radial to the axis of the cable and in another embodiment they dene the arc of' a cylinder, and/or are adjacent to the outer surface of the jacket.

Our improved method of making a cable with drain wires comprises the step of continuously passing an insulated cable core through the die of an extruder wherein 3,582,417 Patented June 1, 1971 ice it isA directly surrounded with a thick layer of semiconducting polymeric material. Concurrently we pay a plurality of drain wires through the die in spaced locations around the core whereby the drain Wires are imbedded in the jacket. The jacket is then cooled, sealing in the drain wires and rmly attaching them to the cable.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a pictorial view of a cable made to our invention.

FIG. 2 shows a partial section of a cable in one embodiment of our invention.

FIG. 3 shows a pictorial view of a section of cable made to another embodiment of our invention.

FIG. 4 illustrates the steps of the method of our invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. l, a cable, indicated generally by the numeral 10 has a conductor 11, strand shielding 12 and wall of insulation 13 of known structure. The insulation 13 may, for example, be thermoplastic polyethylene, synthetic rubber, or cross-linked polyethylene but is not limited thereto. Directly over the insulation and in intimate contact with its surface at all points the insulation is surrounded by a thick jacket 14 of semiconducting polymeric material, which in the illustrated case is polyvinylchloride that has been rendered semiconducting in a known manner by the inclusion of conducting carbon black. Other polymeric materials suitable for the jacket stock of the cable of our invention include, but are not limited to, neoprene, polyethylene, and ethylene-propylene copolymer. Distributed evenly around the circumference of the cable 10, beneath the outer surface of the jacket 14, are six undulatory copper drain wires 15, 16, 17, 18, 19, 20 which can serve as neutral conductors and increase the electrical protection offered by the jacket 14. Because of the undulating shape that has been given to the drain wires 15-20 they do not decrease the ilexibility of the cable 10 even though they are not wound helically around it in the usual manner but are laid in lines parallel to the cable axis. Although the illustrated Wires are copper it will be appreciated that other metals or combinations of metals may also be used within the scope of our invention, and the fact that the drain wires are covered by the substance of the jacket 14 which protects them from corrosion will allow the use of alumimum in underground locations.

It will be appreciated that the contact resistance between the wires 15-20 and the substance of the jacket 14 is minimal and certainly much lower than the resistance between the jacket and wires applied helically over the outside. Such wires have only 1ine, rather than surface, Contact with the jacket and this is reduced by differential expansion and by the imposition of foreign particles, particularly when the cable is buried.

The wires 15-20 in the embodiment of FIG. l are located just beneath the surface of the jacket 14, with the undulations in planes normal to radii through the cable axis. In some cases it may be desired to have the wires 15-20 break through the surface at the ends of the loops of the undulations, such as the points 21, 22. This has the advantage of making the Iwires visible for purposes of grounding and of identifying the outer surface to be conducting rather than insulating. Where it is desired to protect the wires 15-20 more thoroughly from corrosion, they may be embedded furtherbeneath the surface of the jacket 14 and still afford the benefits of our invention. One of these benets, where the cables are buried, is protection from accidental penetration. The undulating disposition of the drain wire can be seen to cause it to protect a greater area, mechanically, than the same wire, helically applied. Electrically, also, the undulatory disposition of the drain wires reduces the maximum current path through the jacket material.

It is apparent, also, from a consideration of FIG. 1 that the undulatory Shape of the wires -20 provides slack for making connections so that a shorter area of cable must be stripped back to provide a length of wire for making a connection than is required where straight drain wires are helically applied.

In the embodiment of FIG. 2 the drain wires, of which wires 23 and 24 can be seen, are disposed lwith their undulations radial to the cable. In this conguration the surface of the jacket may be broken, as shown, by permitting a portion less than a thickness of the wires 23, 24 to extend periodically above the surface where it will be visible.

In the embodiment of FIG. 3 the drain wires have been undulated in arcs of an imaginary cylinder 26 around the cable axis. By this means a line of each of drain wires 27, 28 can be tangent to the surface of the jacket Without breaking the smooth circular section of the cable. Where a plurality of cables are installed in a duct from which one cable may be withdrawn a smooth outer surface provides the least chance of snagging or interlocking of the cable surfaces and this is provided by the construction of FIG. 3 with the maximum area of drain wire adjacent to the surface.

As an example of our invention a cable was constructed to the following dimensions:

Diameter, mils Core consisting of stranded copper conductor, semiconducting strand shielding, and cross-linked polyethylene insulation .890 Jacket-semiconducting PVC 1.050 Drain wires-6 of #19 A.W.G.-copper have undulations between crests of 0.22 inch and a total breadth of .l5 inch The precise dimensions of the corrugations of the drain wires are not critical except that the breadth and frequency of the corrugations should be suicient to allow the cable to bend. In any event, satisfactory corrugations can be sealed from those of the example for cables of diierent dimensions. Neither is it necessary that each of the drain wires should consist of a single lamentory strand, two or more parallel or even crossing strands being serviceable, provided each of them is undulatory. Referring now to FIG. 4 a method of making our cable at high speed and reduced cost comprises the step of paying a cable core 31 through a core tube 32 within an extruder head, of which many suitable types are known and the details of which are not shown. The core tube pays into a die 33 within which the core is subjected to the step of being covered with jacketing compound 34 fed by elements of the extruder, not shown, under high pressure. The core tube 32 has a plurality of lengthwise slots 36 into which are payed a like plurality of drain wires 37 which have been given the desired undulatory shape. By this method of manufacture the drain wires are applied during the extrusion operation at extrusion speeds and the lwires are securely embedded in the semiconducting jacket. Upon leaving the die 33 the cable passes through a cooling apparatus 38, such as a water trough, -where the jacket is subjected to a cooling step where it solidies.

Where it is desired to vulcanize the jacket there may be an intermediate heating step between the die and the cooler 38.

We have invented a new and useful cable and method, of which the foregoing description has been exemplary rather than denitive, and for which we desire an award of Letters Patent.

We claim:

1. The method of making an electric cable comprising the steps of:

l(A) continuously passing an insulated cable core through the die of an extruder and therein directly surrounding said core with a thick jacket of semi- `conducting polymeric material.

(B) concurrently paying a plurality of undulatory drain wires through said die in spaced locations around said core, whereby said drain wires are imbedded in said jacket, and

(C) cooling said jacket, thereby sealing in said drain wires.

References Cited UNITED STATES PATENTS 2,810,669 10/1957 Heupgen 156-51 3,076,235 2/1963 Rollins etal 156-166 VERLIN R. PENDEGRASS, Primary Examiner 

